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The MegaPipe image stacking pipeline

The MegaPipe image stacking pipeline MegaPipe Stacking Procedure

MegaPipe Stacking Procedure

This page describes the procedures used to generate the
stacked images and catalogues for the MegaCam image stacking pipeline.
In short, the procedure is to calibrate each CCD from each exposure of
the MegaCam
mosaic camera to high precision astrometrically and
photometrically, and then add the images together.

Be free from major problems that preclude good calibration
as discussed in the following section.

Quality control

The data are retrieved from the CADC archive.
The images have already been detrended with the
Elixir
pipeline. The images come with a fairly accurate (0.5-1.0
arcsecond) astrometric solution and a photometric calibration.
One CCD of each exposure is inspected visually. Exposures with
obviously asymmetric PSF's (due to loss of telescope tracking) or
other major defects such as terrible seeing, bad focus, or poor
atmospherique transparency are discarded. In some images, one or
more of the CCD's in the mosaic are dead. These images are also
discarded.

Astrometric calibration

The AstroGwyn astrometric calibration pipeline is run on the
images. The first step is to run SExtractor on
each image. The parameters are set so as to extract only the most
reliable objects (5 sigma detections in at least 5 pixels). This
catalogue is further cleaned of cosmic rays and extended
objects. This leaves only real objects with well defined centres:
stars and (to some degree) compact galaxies.

This observed catalogue is matched to the astrometric reference
catalogue.
The (x,y) coordinates of the observed catalogue are
converted to (RA, Dec) using the initial Elixir WCS. The catalogues
are shifted in RA and Dec with respect to one another until the best
match between the two catalogues is found. If there is no good match
for a particular CCD (for example when the initial WCS is
erroneous), its WCS is replaced with a default WCS and the matching
procedure is restarted. Once the matching is complete, the
astrometric fitting can begin. Typically 20 to 50 sources per CCD
are found with this initial matching.

Elixir provides a first order solution for the WCS with typical
errors on the order of 1 arcsecond. AstroGwyn improves on this to
provide a higher order solution with an accuracy of typically 0.1
arcseconds. As the accuracy of the WCS improves, the observed and
reference catalogues are compared again to increase the number of
matching sources. A larger number of matching sources makes the
astrometric solution more robust against possible errors (proper
motions, spurious detections, etc.) in either catalogue.

The higher order terms are determined on the scale of the entire
mosaic. That is to say, the distortion of the entire focal plane is
measured. This distortion is well described by a polynomial with
second and fourth order terms in radius measured from the centre of
the mosaic. The distortion appears to be stable over time, even when
some of the MegaPrime optics are flipped. Determining the distortion
in this way means that only 2 parameters need to be determined (the
coefficients of r2 and r4) with typically (20-50
stars per chip) * (36 chips) =~ 1000 observations. If the analysis is
done chip-by-chip, a third order solution requires (20 parameters per
chip)*(36 chips)= 720 parameters. This is less satisfactory.

From the global distortion, the distortion local to each CCD is
determined. The local distortion is translated into a linear part
(described by the CD matrix) and a higher order part (described by
the PV keywords). The CD/PV
transformation was described in detail in an appendix that was
removed from first draft of the MegaPipe paper.
The higher order part is 3rd order as well, but the coefficients
depend directly and uniquely on the 2 parameter global radial
distortion. The error introduced by this translation is less than
0.001 arcseconds.

For the first band to be reduced (the i-band, if it
exists, otherwise the order of preference is r, g, z, u),
these source catalogues are matched with the an external
astrometric catalogue to provide the initial astrometric solution.
If available, the SDSS catalogue is used, otherwise
the 2MASS catalog is
used.

For the other bands, the observed catalogues are first matched to
the external catalogue and then matched to a catalogue generated
using the first image in order to precisely register the images in
the different bands. The final astrometric calibration has an
internal uncertainty of about 0.03 arcseconds and an external
uncertainty of about 0.1 arcseconds, as
discussed on the checks on astrometry page.

Photometric calibration

The Sloan Digital Sky Survey DR9 serves
as the basis of the photometric calibration.
The Sloan ugriz filters are not identical to the MegaCam filters.
The colour terms between the two filter sets
can be described by the following equations:

The relations for the griz bands come from the analysis of the
SNLS group.
The relation for the u band comes from the
CFHT web pages.

All images lying in the SDSS can be directly calibrated without
referring to other standard stars such
as Smith standards. The systematic uncertainties in the SDSS
photometry are about 0.02 magnitudes
(Ivezic, et al., 2004).
The presence of at least 1000 usable sources in each
square degree reduces the random error to effectively zero. It is
possible to calibrate the individual CCDs of the mosaic individually
with about 30 standards in each.
For each MegaCam image, the observed
catalogue is matched to the SDSS catalogue for that patch
of sky.
The difference between the instrumental MegaCam magnitudes
and the SDSS magnitudes (transferred to MegaCam system using the equations
above) gives the zero-point for that exposure
or that CCD. The zero-point is determined by median, not mean.
There are about 10000 SDSS sources per square degree, but
when one cuts by stellarity and magnitude this number drops to
around 1000.
It is best to only use the stars (the above colour
terms are more appropriate to stars than galaxies) and to only use the objects
with 17<mag<20 (the brighter objects are usually saturated
in the MegaCam image and including the fainter objects
only increases the noise in the median).
This process can used for data from any night. It is not necessary for the
night to be photometric.

For objects outside the SDSS, the Elixir photometric keywords are
used, with modifications. The Elixir zero-points were compared to
those determined from the SDSS using the procedure above for a large
number of images. There are systematic offsets between the two sets of
zero-points, particularly for the u band. These offsets show
variations with epoch, which are caused by modifications to Elixir
pipeline. There also differential offsets between the CCDs of a
single image. For MegaPipe, the offsets are applied from the Elixir
zero-points to bring them in line with the SDSS zero-points. A
detailed analysis of these offsets has been made.

The Elixir photometric keywords are only valid on photometric
nights. Archival data from the SkyProbe
real-time sky-transparency monitor is used to determine if a night
was photometric or not. Data taken on photometric nights is
processed first through the astrometric and photometric pipelines
to generate a catalogue of in-field standards. These standards are
then used to calibrate any non-photometric data in a group. If
none of the exposures in a group was taken on a photometric night,
that group cannot be processed.

Coaddition

The calibrated images were coadded using the program
SWarp. Here
is the SWarp configuration file.
The resulting stacks are simple FITS files
(not multi-extension FITS files) measuring about 20000 pixels by 20000 pixels
or about 1 degree by 1 degree, depending on the input dither pattern,
and are about 1.7 Gb in size.
They have a sky level of 0 ADU. They are scaled to have a photometric
zero-point of 30.000 in AB magnitudes - that is to say, for each source:

AB_magnitude = -2.5 * log10(ADU) + 30.000

A weight map (inverse variance) of the same size is also produced.

Photometric catalogue

SExtractor is run on each stack using the weight map.
Here is the SExtractor configuration file.
The resulting catalogues only pertain to a single band image; no
multi-band catalogues have been generated.
While this fairly simple approach works well in many cases,
it is probably not optimal in some situations.
Depending on the application,
some users may wish to run their own catalogue generation
software on the stacks.